There are prior art systems for weighing containers which consist of resting the container to be weighed on a plate of a weighing cell and leaving it for a period of time (as a function of the characteristics of the weighing cell and the required accuracy for the measurement), at the end of which the weighed container is moved away towards successive stations/process steps.
In such a weighing system, the containers are moved in step mode and the weighing time between one step and the next. As the weighing time increases, and therefore the accuracy which can be obtained increases, the productivity necessarily falls.
There are also prior art weighing systems which comprise moving the containers continuously and the weight of the containers is measured in an indirect manner by sensors of various types, such as capacitive, microwave, X-ray sensors etc. These indirect weighing systems have the advantage of allowing high productivity, but not equally guarantee measurement precision and accuracy.
In the technical sector of filling capsules for extraction or infusion beverages, the capsules, used in machines for making these beverages, comprise in their simplest form, the following:                a rigid, cup-shaped container comprising a perforatable or perforated bottom and an upper aperture provided with a rim (and usually, but not necessarily, having the shape of a truncated cone);        a dose of extraction or infusion beverage product contained in the rigid container;        a closing piece (or element) obtained from a web for sealing the aperture of the rigid container and designed (usually but not necessarily) to be perforated by a nozzle which supplies liquid under pressure.        
Preferably, but not necessarily, the sealing sheet is obtained from a web of flexible material. In some cases, the capsules may comprise one or more filtering elements of the rigid or flexible type.
For example, a first filter (if present) may be located on the bottom of the rigid container. A second filter (if present) may be interposed between the piece of sealing sheet and the product dose.
The capsule made up in this way is received and used in specific slots in machines for making beverages.
Patent document WO2013/121319A1 in the name of the same Applicant as this invention describes a machine for packaging single-use capsules having an element for conveying cup-shaped containers which extends along a closed horizontal path, for conveying the cup-shaped container between the various processing stations, in which cup-shaped container is filled and closed.
The processing stations are positioned along the closed path, for allowing the filling of a rigid, cup-shaped container with a dose of product and closing the container with the closing element.
A strongly felt need in the machine is that of weighing, in a particularly reliable and quick manner, the rigid, cup-shaped containers filled and closed with doses of product.
More specifically, a strongly felt need is that of weighing the rigid, cup-shaped containers filled and closed at high operating speeds, that is to say, without requiring an intermittent operation of the machine.
For this reason, the need has been found of having a unit for weighing closing elements which can be associated with the machine according to patent document WO2013/121319A1.